Conventional photolithography involves projecting actinic radiation onto a photoresist clad wafer. The physical limit to the numerical aperture (NA) for such exposure systems using air as a medium between a lens and the wafer is 1. NA is actually determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens. Microscopy has for some time employed oil for resolution enhancement.
Immersion technology offers improved resolution enhancement and higher numerical apertures over conventional projection lithography. In immersion lithography, the space between the projection lens and the wafer in a tool is filled with a liquid. That is, immersion lithography uses a thin layer of liquid to further focus resolution. However, immersion lithography typically requires large, expensive lenses.
While the immersion is promising, there are a number of concerns associated with implementing immersion lithography that require resolution in order for the technology to gain wide acceptance. For example, there is a tendency for the liquid in immersion lithography to develop annoying micro-bubbles, which rains the benefits offered by the technology. Maintaining a consistent bubble free liquid between the lens and the wafer is very difficult. Polarization of the lens is also a significant concern.
Water is the most common liquid employed in current immersion lithography systems. This is because water has an index of refraction of about 1.47, an absorption of less than about 5% at working distances of up to 6 mm, is compatible with most photoresists and lens, and in an ultrapure form, is non-contaminating. Specifically, the liquid employed for most immersion applications is double deionized, distilled, degassed water. Nevertheless, improvements in immersion lithography are desired.